The densification of silicon nitride to a substantially full density anisotropic material has usually been carried out by hot pressing of a porous silicon nitride body under sufficient pressure and temperature conditions to consolidate the body to full density (see U.S. Pat. Nos. 3,835,211; 4,179,301; 4,099,979). This mode is essentially a batch process requiring cooling of the hot pressing fixture before reloading for a subsequent hot pressing cycle.
The porous silicon nitride body has been prepared by nitriding a silicon powder in a nitrogen bearing atmosphere at sufficient temperature to form silicon nitride. The pressing additives or nitriding catalyst agents may be added to the silicon powder prior to nitriding or the pressing aids can be added to the nitrided powder following the nitriding operation after it has been reground back to a powder. Furthermore, the consolidation of the silicon nitride powder can be promoted either by cold pressing the silicon powder with its additives prior to nitriding or the nitrided powder, with subsequent additions, may be cold pressed following the nitriding operation.
Regardless of the prior art method employed, the density of the porous silicon nitride body following nitriding and cold pressing has been in the range of 2.3-2.6 gm/cm.sup.3. This is considered a relatively low density range and dictates a high compression ratio and a long compression stroke to achieve full density during hot pressing.
To increase economy of processing, several porous bodies may be simultaneously hot pressed in a stack in the same cavity. This exaggerates the high compression ratio even further, requiring a still longer compression stroke. This has resulted in distortion in the densified products because of a nonuniform pressure distribution across the individual bodies. The nonuniform pressure distribution results from the existence of a temperature gradient and an accompanying viscosity gradient across the lateral width of a body. A frictional drag force (experienced between the sides of the bodies and the walls of the hot pressing assembly), in conjunction with the nonuniform pressure distribution, causes material transport within the bodies under hot pressing conditions which results in "dishing" or a severe form of distortion in the fully densified bodies.
This problem has been solved in part by my invention disclosed in U.S. application Ser. No. 444,246, in which rigid, inert spacers are inserted into the stack of low density bodies at predetermined multiples of the bodies. However, use of spacers reduces the available amount of body material that can be subjected to the effective hot zone of the hot pressing cavity. This not only results in a reduction of process efficiency, but lengthens the cycle time of hot pressing an equivalent number of bodies.
The disadvantages of following this prior art is the inefficient use of the existing hot zone and considerable wear and tear on hot pressing apparatus.
What is needed is a method in which 100% of the effective hot zone is occupied by preformed bodies during hot pressing and which bodies are densified at low pressures, shorter times, and lower compaction ratios.